The majority of Ultra High Frequency (UHF) antennas used in National Television System Committee (NTSC) antenna systems are slotted coaxial designs. UHF slotted coaxial antennas gained widespread use in NTSC broadcasting because of their above-average performance characteristics; namely, excellent omni-directional azimuth patterns, low wind loads, and smooth null fill.
While the foregoing performance characteristics are also desirable for digital television (DTV) transmission, the more stringent antenna output performance standards of DTV transmission cannot be met with current slotted coaxial antenna designs. At the present stage of antenna development, the antenna output response performance across multiple channels, which was given little consideration in NTSC systems, is now an important parameter for DTV transmission.
For example, for NTSC transmission, the power distribution across a six MHz television channel is concentrated at three basic carrier frequencies; namely, picture, color and aural. Therefore, the performance of the antenna is critical only at these three carrier frequencies.
However, for DTV transmission, the power is equally distributed across a 5.4 MHz frequency span within the 6 MHz channel. Therefore, the antenna's performance is critical across the entire operating band. This means that the antenna's elevation pattern must remain stable (i.e. unchanged) at all frequencies within the channel, and not just at isolated frequencies.
Use of coaxial antennas for DTV transmission is therefore hindered by the fact that slotted coaxial antennas are not suitable for multi-channel applications. This is due in part to the fact that the slots are not broadband radiators. It is also due to the linear "tap off" feed style, which is conventional practice for slotted coaxial antennas.
As shown in Prior Art FIG. 1, slots 2 are cut axially along the outer conductor 4 and fed from one end 6 in a typical feed design of a slotted coaxial antenna. The spacing "X" of all of the slots along the outer conductor 4 is selected to match with the frequency of a particular channel. Thus, this antenna will exhibit a progressive phase taper as the input frequency varies. This progressive phase taper causes the elevation pattern to change with frequency and, thus, limits the useable bandwidth of the antenna.
As in the prior art antenna of FIG. 1, many slotted coaxial antennas are designed to feed the radio frequency (RF) power from the bottom end of the antenna (i.e., end-fed design). Mechanically, end-fed antennas are convenient particularly for antennas mounted on the top of towers. The signal is fed from the bottom and travels toward the top.
Since signals are fed from the bottom traveling toward the top, the end-fed antenna is constructed with a nominal one wavelength spacing between slots at the design frequency. In theory, as the signal moves from one slot to the next upward along the antenna, a 360-degree phase rotation occurs in effect placing each successive slot level in phase. However, in reality, the one wavelength spacing is only obtained precisely at the design frequency. As a consequence, when the signal frequency scans above or below the exact design frequency, the phase rotation changes, causing the beam tilt to vary. The undesirable effect of beam tilt variation is a detrimental impact on antenna output performance response, of major importance for DTV.
Conventional wisdom that the use of panel antennas to solve the NTSC/DTV problem of multi-channel capability is also not a viable solution. Panel antennas are able to operate over a very wide band by using broadband radiators (typically dipoles) in conjunction with branch feeding each individual panel by power dividers and feedlines. However, a major disadvantage of these antennas is their high wind loading. Panel antennas have larger flat surfaces and thus exhibit much higher wind load than slotted coaxial antennas. Moreover, reliability is an issue with panel antennas due to the complexity of the feed systems, which involve numerous feedlines and connections.
A different slotted cylinder feeding design is the center-fed design, where the signal is fed into the middle of the antenna where the feed point is located. Prior Art FIG. 2 illustrates this design, generally used in a side-mount antenna. As shown in FIG. 2, center feeding is accomplished by using an input "T" 8 between the two antenna halves 10. The advantage of center fed antennas is that the signal travels outward from the center in both directions. The resultant phase taper across the entire aperture of the antenna is therefore zero. The beam sway associated with frequency change is thus eliminated.
In a top mount antenna, as shown in Prior Art FIG. 3, center feeding is mechanically more complex. It requires using a tri-axial configuration 12 in the lower half of the antenna. The triax adds to the mechanical complexity because the inner of the coax at the input becomes the inner for the antenna top section, and the outer of the coax at the input becomes the inner for the antenna bottom section.
The onset of DTV has thus complicated the antenna selection decision for broadcasters who must now operate DTV antenna systems simultaneously with their existing NTSC antenna systems. It would be desirable therefore to provide a coaxial antenna for the DTV and NTSC antenna systems that exhibits acceptable signal coverage, minimal tower wind loading and center feeding, even in top mounted antennas.